The present invention relates to a process for preparing overbased calcium sulfonates or calcium sulfonates having a high degree of alkalinity.
Within internal combustion engines working under strong load and using fuels which contain sulfur, sometimes in relatively high amounts, serious problems arise, especially with regard to lubrication. The strong acidity produced by the combustion of the fuel induces a corrosion of the engine and an intense oxidation of the lubricating oil which leads to a formation of varnish and sludge.
One preferred way to counteract these detrimental effects is to incorporate a calcium sulfonate having a high degree of alkalinity into the lubricating oil.
The detergent effect of the calcium sulfonates is well known for a long time, and, U.S. Pat. No. 2,270,577 may be cited as reference in this field.
The degree of alkalinity or in other words the potential alkalinity of the calcium sulfonate serves to neutralize the strong acidity to which the lubricating oil is exposed during its use and to avoid or at least to strongly reduce the detrimental effects of that acidity. This action of overbased sulfonates is also known for a long time (see for instance U.S. Pat. Nos. 2,418,894; 2,402,325; 2,485,861 and 2,501,731).
In general the process of preparing oils which contain overbased calcium sulfonates essentially consists of reacting a solution of alkylbenzene sulfonic acids having a molecular weight higher than 400, in oil with calcium oxide or hydroxide and bubbling carbon dioxide through the reaction mixture; thereby an incorporation of an excess of calcium carbonate into the calcium sulfonate is effected which confers to the product a potential alkalinity.
The first products which were obtained in this manner had only a low potential of alkalinity; moreover, they formed more or less cloudy solutions which were difficult to filter because they caused clogging of the filters. Thereafter, the methods have been improved and the potential of alkalinity could be increased up to values sometimes reaching 400 mg KOH/g. Nevertheless, the problems which are encountered during the filtration of these products remains the most limiting factor of their preparation. The efforts to improve that filtration have been obviously directed to additives which can improve the reaction of carbon dioxide with calcium oxide or hydroxide. For this purpose alcohols, amines or ammonia are frequently added to the reaction mixture. It has also been thought that the difficulties of filtration could be due to impurities of the calcium oxide, which are present in the commercial quicklime. Therefore it has also been tried to use quicklimes as reactive as possible. However, these different factors have only a small influence on the filtration step which remains the most critical step of the process.